1. Field of the Invention
The present invention relates to low intensity level luminairies, particularly for use marking the location of doorbell buttons, driveway edges and the like, and more particularly relating to a battery powered luminaire providing a useful battery life of one or more years.
2. Description of the Problem
Since the introduction of wireless doorbells, customers have requested a lighted button feature to assist in locating the doorbell button in the dark. Lacking connection to line electrical power, providing such a feature has proven impractical to achieve with even the smallest incandescent sources. The power demands of incandescent bulbs exhaust the capacity of typical battery sizes usable in these products within hours, or days, at best. Larger batteries could increase battery life, but these are costly and their bulk is not appropriate in the application of a doorbell button. Early, non-high intensity type, LED light sources, while operable for far longer periods than incandescent sources, still cannot operate at the very low current levels required to obtain desirable battery life objectives of one year or longer while emitting useful levels of light.
Other products could benefit from a battery powered, long life light source suitable for use in a wireless doorbell. Self-contained battery powered chimes hardwired to a door mounted push button are very common in Europe, although somewhat rare in North America. Lighted buttons are a desirable feature here as well, but cause the batteries in the chime to become quickly exhausted. Thus battery chime systems have not included a lighted button. Presently, incandescent bulbs and low efficiency LED light sources are used in lighted buttons, but they consume far too much current to provide acceptable battery life in battery powered chimes.
Battery life can be extended for an LED device by causing the LED to blink on and off. This can also serve to attract attention to the device. For a residential application however, most consumers do not want to have a blinking LED marking their doorbell, driveway, or sidewalk. Operating the LED on a continuous basis may be more attractive to consumers, but would require substantially more power.
Reflector based markers and some types of landscape lights could also benefit from a long life battery powered luminaire. Roadside, bicycle and driveway reflector products are very effective when a bright source of light shines directly on them. Otherwise, such reflectors are ineffective. A self-lighted marker has the advantage of being visible without an external source of light directed on it, so that it is visible to walkers, joggers and bicyclists at night. Such a marker would also be useful in driving situations where the marker is outside the normal field of the car's headlights. Roadside reflectors have been proposed that have made use of solar charging systems for batteries. Rechargeable batteries are bulky and the solar cells and recharging circuits can add substantially to the relative cost of the product. Solar cells must be placed in locations that receive direct sunlight during some part of the day, and, as a consequence, may not work in a shaded location. During winter at high latitudes very little sunlight is received, reducing the effectiveness of these products.
Under conditions of darkness, it does not require much light output to make an object visible. The human eye has great light intensity adaptability. The differences in eye sensitivity between conditions of bright sunlight (photopic vision) and fully night adapted vision (scotopic vision) can vary by a factor of 25,000 and instances of adaptation up to a factor of 1,000,000 times have been documented. Multiple mechanisms within the eye provide this adaptability, some responding quickly to changing light conditions, e.g. pupil dilation, and some slowly, e.g. maximum rod sensitivity, so that fully night adapted vision is not achieved for up to 30 minutes. The implication of this is that levels of light useless under normal indoor lighting conditions, can become useful under conditions where one can anticipate people will have adapted to darkened conditions. The spectrum of light generated makes a difference in the minimum radiant intensity required for human perception. Generally people can see broad spectrum or white light more readily than they can see narrow spectrum light of the same intensity.
Visible spectrum applications of light emitting diodes have long included simple status indicators and dynamic power level bar graphs. Display applications have grown in number and super bright LEDs are used in various automotive and traffic signal applications. Super bright LEDs are extremely efficient in terms of the percentage of input power converted to visible radiation compared with devices previously known. This is one reason they are favored for applications requiring the output of high intensity light. Super bright LED devices are available which emit any one of a variety of colors, or which emit broad spectrum radiation. Some super bright LEDs also work over broad ranges of drive currents and emit low intensity light at low drive currents and with low power consumption. These LEDs exhibit efficiencies at low power levels comparable to the high efficiencies achieved at the much higher power levels at which they are designed to operate. U.S. Pat. No. 6,140,776 to Rachwal teaches a flashlight that exploits low power operation of super bright LEDs in one application.